Thermally responsive materials based on crystalline colloidal arrays have been used for a variety of purposes. A crystalline colloidal array (CCA) is a three-dimensional ordered array of mono-dispersed colloidal particles. The particles are typically composed of a polymer latex, such as polystyrene or an inorganic material, such as silica.
Such colloidal dispersions of particles can form crystalline structures having lattice spacings that are comparable to the wavelength of ultraviolet, visible, or infrared radiation. These crystalline structures have been used for filtering narrow bands of selected wavelengths from a broad spectrum of incident radiation, while permitting the transmission of adjacent wavelengths of radiation. Prior devices have been created by dispersing particles in a liquid medium, whereby the particles self-align into an ordered array. The particles may be fused together by mutual polymerization or by introducing a solvent that swells and fuses the particles together.
In other uses of CCAs, an ordered array is fixed in a matrix and may be used to exhibit color when the fixed array diffracts radiation in the visible spectrum. Alternatively, CCAs are fabricated to diffract radiation for use as optical filters, optical switches and optical limiters. While these CCAs use constant interparticle spacing, other CCAs may be thermally active whereby the interparticle spacing varies in response to stimuli, such as a temperature change.
Thermally responsive CCAs conventionally have been produced from hydrogels as a matrix material with a CCA embedded therein. In hydrogel-based devices, mono-dispersed, highly-charged colloidal particles are dispersed in an aqueous media. The particles self-assemble into a CCA due to their electrostatic charges. These ordered structures diffract radiation according to Bragg's law, wherein the radiation meeting the Bragg conditions is reflected while adjacent spectral regions that do not meet the Bragg conditions are transmitted through the device.
An ordered periodic array of particles that diffracts radiation according to Bragg's law satisfies the equation:mλ=2nd sin θwhere m is an integer, λ is the wavelength of reflected radiation, and n is the effective refractive index of the array, d is the distance between the layers of particles, and θ is the angle that the reflected radiation makes with the plane of a layer of particles. Incident radiation is partly reflected at an uppermost layer of particles in the array at angle θ to the plane of the first layer and is partially transmitted to underlying layers of the particles. While some absorption incident radiation occurs as well, a portion of the transmitted radiation is partially reflected at the second layer of particles in the array at angle θ and partially transmitted to the underlying layers of particles. This feature of partial reflection at angle θ and partial transmission to the underlying layers of particles continues through the thickness of the array. The wavelength λ of diffracted radiation can be controlled by the dimension d, which may be the distance between the planes of the centers of the particles in each layer. Initially, the diffracted wavelength λ is proportional to the particle diameter for an array of packed particles. However, when distance d between layers of particles in a periodic ordered array increases, the wavelength of diffracted radiation also increases. By increasing the particle size or the volume of the matrix in response to a stimulus, the inter-particle distance between layers of particles increases, thereby altering the wavelength of diffracted radiation.
In a hydrogel-based CCA, the particles and/or the volume of the hydrogel matrix changes in response to temperature changes, thereby changing the diffraction wavelength of the CCA. Such CCA devices that are based on hydrogels typically contain a large percentage of water, i.e., at least about 30% by volume of water. For hydrogel-based CCAs, the colloidal particles and/or the hydrogel matrix are composed of a material that undergoes a change in its spatial dimensions in response to a temperature change. The spatial dimension change may be due to dehydration of the hydrogel particles and/or hydrogel matrix at elevated temperatures or expansion of the water within the hydrogel CCA at reduced temperatures. As such, hydrogel CCAs are functional only within a limited temperature range, the temperature at which water is liquid. These hydrogel CCAs are also prone to dehydration from evaporation of the water, even at room temperature. Such dehydration alters the inter-particle dimensions, creating instability in the hydrogel CCA.